Behavioural response to interference competition in a sessile suspension feeder

The sessile suspension-feeding worm-snail Dendropoma maxima Sowerby, 1825 (Vermetidae) secretes a mucous web to capture planktonic prey. In dense groups, the feeding webs of neighbouring snails frequently overlap and stick together. This may create direct food competition between neighbours because the earlier retracting snail may get more than its fair share of the prey. While field observations indicate that web overlap may generate retarded growth, we experimentally studied whether web overlap also triggers a phenotypic response in feeding behaviour. In our experiment we consecutively placed focal snails in a place by themselves (solitary) or close to a neighbouring snail such that webs overlapped, starting with either of the 2 conditions in half of the experimental individuals to exclude sequence effects. We found that focals retracted their feeding web significantly earlier when close to a neighbour than when solitary. Our experiments thus confirm a phenotypic response through early web retraction in D. maxima, indicating that direct interference competition affects worm-snail behaviour.